196 INDIVIDUALITY OF CHROMOSOMES [CH. 



synizesis stages of the maturation of the pollen grains, when 

 they become "spun out" into long threads that are not 

 separable as individuals. And several investigators, in 

 attempting carefully to follow the chromosomes from one 

 division to the next have thrown important light on the 

 nature of the nuclear network in the so-called resting stage. 

 The work of BOLLES LEE (1912) on the development of the 

 pollen-grains of the plant Paris quadrifolia^ and that of 

 WILSON (1912) and others on the spermatogonia of several 

 insects, may be taken as examples. BOLLES LEE finds that 

 at the close of the second maturation division of the pollen 

 grains the chromosomes first become vacuolated and then 

 gradually elongate into spiral threads, and that with suffi- 

 cient magnification the spirals are still recognisable in the 

 resting nucleus. The apparent reticulum of the resting 

 nucleus is in fact a number of spiral threads, to some extent 

 intertwined, but evidently retaining their individuality and 

 each derived from a single compact chromosome of the pre- 

 ceding anaphase stage. In plants there is a nuclear division 

 in the pollen grain after the second maturation division, 

 and when this approaches, the fine spirals are seen to con- 

 tract and thicken and to become vacuolated as they were 

 in the preceding telophase. The vacuoles form a row along 

 the centre of the chromosome, and mark the line along which 

 the longitudinal division will take place. After the division 

 is completed, the same process of forming elongated spirals 

 can again be seen, and in the two resting nuclei of the 

 mature pollen grain, the structure which on superficial ex- 

 amination appears as a reticulum is found when examined 

 carefully to consist of these fine spiral threads. 



KRISTINE BONNEVIE has drawn similar, but not identical, 

 conclusions from her work on Ascaris, Allium (the Onion) 

 and Amphiuma. She finds that within the chromosomes of 



